This invention relates to the radio frequency filtering art in general and, more particularly, to a filter network comprising generally a bandreject or notch-type filer in combination with a resonant cavity.
Bandreject cavity filters customarily comprise an odd quarter wavelength resonator connected in shunt to the transmission line. They are tuned so that at the junction between the transmission line and cavity filter a short circuit condition exists for the frequency to be rejected. This short circuit condition exists for a relatively narrow band of frequencies; therefore, the rejection notch is relatively sharp and attenuates a narrow band of frequencies compared to a broad band of frequencies of low attenuation. The broad band of frequencies of low attenuation are on either side of the rejection notch so that the selectivity provided to a radio system by a filter of this type is limited essentially to the narrow area of the rejection notch.
Filters of this type are commonly used in the radio communications field, not only as a filter to attenuate discrete frequencies, but also as interconnected groups of filters for the purpose of multi-coupling. A frequently applied type of multi-coupler is the duplexer which allows simultaneous operation on one common antenna of two pieces of equipment operating at different but closely spaced frequencies. Usually the two pieces of equipment are a paired radio transmitter and receiver. In such a case, the duplexer allows energy at the transmitter frequency to flow freely from transmitter to antenna. Likewise, the duplexer allows energy at the receiver frequency to flow freely from antenna to receiver. At the same time, energy flow from the transmitter to the receiver is impeded; and energy received at the antenna is diverted from the transmitter.
For these purposes, the duplexer comprises a number of filters spaced along the transmission line which connects the transmitter to the common antenna terminal, and a number of filters spaced along another transmission line which connects the common antenna terminal to the receiver. The filters on the transmitter side highly attenuate transmitter noise at the receiver frequency and at the same time pass with low attenuation the transmitter carrier. Filters of the prior art generally reject only a narrow a band of energy centered at the receiver frequency. It would be highly desirable for these filters to attenuate not only transmitter noise at the receiver frequency but also transmitter noise over as wide a frequency range as possible. This characteristic would provide not only further protection for the associated receiver but also protection to other receivers in the area which may be operating in nearby frequency ranges.
The filters on the receiver side pass the receiver frequency with low attenuation and at the same time highly attenuate the transmitter carrier frequency. Filters of the prior art generally reject only a narrow band of signals centered at the transmitter frequency. It would be highly desirable for these filters to attenuate not only the transmitter carrier but also other spurious frequencies over as wide a range as possible. This characteristic would provide not only further protection from the associated transmitter but also from other transmitters in the area operating in nearby frequency ranges.
The transmitter carrier frequency and the receiver frequency must be spaced apart enough to allow the duplexer to adequately isolate the receiver from the transmitter noise spectrum falling within the pass band of the receiver. At the same time, it would be desirable to position the carrier and receiver frequencies as close together as possible in order to narrow the band width required by a multi-coupled radio transmitter and receiver system, thereby making it possible for more systems to operate within a specified frequency range in a given area. It is to this end that minimum isolation between the transmitter frequency and the receiver frequency across a given frequency span becomes important. As the frequencies come closer together, more of the transmitter noise spectrum does fall within the pass band of the receiver to adversely affect reception.